For example, in a lens region of a progressive-addition lens, there are a region of a distance portion for distance vision, a portion for near vision, and a region of an intermediate vision for viewing objects at intermediate distances. A positive refractive power is progressively added from the region of the distance portion through the region of the intermediate vision to the region of the near portion. The difference in refractive power between the region of the distance portion and the region of the near portion of the progressive-addition lens is called an addition power. For a person wearing spectacles for presbyopia who has insufficient power of accommodation in near vision, the addition is designated in accordance with the degree of presbyopia so as to allow comfortable near vision by complementing the power of accommodation. Typically, the addition power of a progressive-addition lens is designated at a value in a range from 0.25 D to 4.00 D by increments of 0.25 D in accordance with the degree of presbyopia and/or the near working distance.
The order information must include an addition power if the order is placed by an optician for a progressive-addition lens with a spectacle lens manufacturer. As described above, the progressive-addition lens has the refractive power varying from the region of the distance portion through the region of the intermediate vision to the region of the near portion in accordance with the designated addition power. Therefore, when the progressive-addition lens is used as a spectacle lens, the size of an object to be viewed varies in respective visual regions of the lens, which causes a person wearing the spectacles to feel “sway” or “distortion”. The “sway” and “distortion” of the progressive-addition lens tend to increase with an increase in the addition power. To make the “sway” and “distortion” small enough, various attempts have conventionally been proposed to decrease as much as possible the “sway” and “distortion” of lenses with the same addition power by devising the refractive power distribution of the progressive-addition lens (see, for example, Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-66148